In integrated circuits the necessity often arises of generating a stable reference voltage.
Band-gap reference voltage generators are known for providing a reference voltage which is insensitive to temperature variations. The general principle on which band-gap reference voltage generators operate is one of cancellation of temperature coefficients: generally, a voltage is developed that is a scaled value of V.sub.T (the so-called "thermal voltage" equal to KT/q, where K is the Boltzmann constant, T is the temperature in Kelvins, and q is the electron charge in absolute value). This scaled value has a well-defined temperature coefficient, that is the scaling constant times the temperature coefficient of V.sub.T, which is positive. Said voltage is added to a base-emitter voltage, which in turn has a negative temperature coefficient. The scaling factor is chosen so that the sum of the temperature coefficients is zero. If the output voltage of the circuit is taken in such a way to contain the sum of the scaled value of V.sub.T and of the base-emitter voltage, the temperature coefficient of the output voltage is zero.
Several kinds of band-gap reference voltage generators are known. The most common type comprises an operational amplifier in negative feedback configuration, with the non-inverting and inverting inputs coupled to respective circuit branches which are also connected to the output of the operational amplifier, each branch containing respective base-emitter junctions of bipolar junction transistor.
In some applications, requiring a low power consumption operating mode, the band-gap reference voltage generator must not dissipate power in said operating mode. This is for example the case of 3 V only Flash EEPROM devices when operated in stand-by. For these devices, the power consumption in stand-by condition is required to be strictly zero, and is therefore necessary to turn all the possible sources of consumption off. Whenever it is necessary to have the reference voltage available, it is necessary to wait for the reference voltage generator power up: the power-up delay is typically in the range of microseconds.